This invention relates to a focus detecting apparatus used in an optical recording and playback system wherein a light is focused and a spot of light is made through an objective lens onto an information-carrying medium to read or write information, the function of the focus detecting apparatus being to detect when the light is not correctly focused on the information-carrying medium.
Optical playback and recording systems have become familiar devices, employing such information-carrying media as compact discs, video discs and optical memory discs. Consider, for example, a video disc system. The information is reproduced from a disc that rotates at high speed while a beam from a laser light source such as a semiconductor laser is focused through an objective lens onto an information track by detecting the modulated light transmitted through or reflected from the information track. A feature of this type of information-carrying medium is the extremely high density of the recorded information: the information tracks are very narrow, and the pitches between adjacent information tracks are very small. If information is to be read accurately from such narrow, closely-spaced information tracks, the objective lens must maintain correct focus on the video disc surface and create a small-diameter light spot having high resolution with respect to the information on the disc surface. To maintain the correct focus, optical playback and recording systems of the this type must detect the departure from focus of the disc surface and correct the focus by moving the objective lens parallel to its optic axis in response to a focusing error signal from a photodetector.
FIG. 6 is a ray-tracing diagram showing the configuration of the principal parts of the focus detecting apparatus of an optical playback and recording system described in Japanese Patent Application Laid-open No. 7246/1981. The beam from a laser light source 1 (which is linearly polarized in the plane of the page) is collimated by a collimator lens 2. Also shown in the drawing are a polarization beam splitter 3, a quarter-wave plate 4, an objective lens 5, an information-carrying medium 6 with information tracks, a detector prism 7 having a reflecting surface 8, and a photodetector 9 comprising two sections 9A and 9B.
This apparatus operates as follows. The beam (which is linearly polarized parallel to the plane of the page) from the laser light source 1 is collimated by the collimator lens 2, transmitted through the polarization beam splitter 3 and the quarter-wave plate 4, and converged by the objective lens 5 onto the information-carrying medium 6 containing the information tracks. In FIG. 6 the information tracks run perpendicular to the page, but they could also run parallel to the page, in the left-right direction in the drawing. The beam focused onto the information-carrying medium 6 is reflected and returns through the objective lens 5 and the quarter-wave plate 4 to the polarization beam splitter 3.
In the configuration shown in the drawing, due to the action of the quarter-wave plate 4, the light incident on the polarizaton beam splitter 3 is polarized perpendicular to the plane of the page, so it is reflected by the polarization beam splitter 3 (through an angle of 90.degree. to the left in the drawing). The beam reflected from the polarization beam splitter 3 enters the detector prism 7 and is reflected from its reflecting surface 8, then is received by the photodetector 9.
The reflecting surface 8 is set so that when the objective lens 5 is correctly focused with respect to the information-carrying medium 6, the angle between the reflecting surface 8 and the incident beam (a parallel pencil of rays in this case) is equal to or slightly less than the critical angle. If it is exactly equal to the critical angle, in the correctly focused state the entire beam reflected from the polarization beam splitter 3 undergoes total reflection at the reflecting surface 8. Since the reflecting surface 8 is necessarily imperfect, however, some light is also channeled in direction n in the drawing. If the information-carrying medium deviates from the point of focus in direction a in the drawing, the beam reflected from the polarization beam splitter 3 will include an oblique component, the maximum angle of obliquity of which is indicated by a.sub.i1 and a.sub.i2. If the information-carrying medium deviates from the point of focus in direction b in the drawing, the beam incident on the reflecting surface 8 will include an oblique component, the maximum angle of obliquity of which is indicated by b.sub.i1 and b.sub.i2. In either case, if the information-carrying medium 6 deviates from the point of focus, the beam incident on the reflecting surface 8 varies continuously around the critical angle, except for the central ray on the optic axis (the dash-dot line in the drawing). The reflectivity of the reflecting surface 8 is extremely sensitive to slight changes in the angle of incidence in the neighborhood of the critical angle, as indicated in FIG. 7. When the information-carrying medium 6 deviates in direction a or b from the point of focus, the intensity of the reflected beam will be less on one side of the plane perpendicular to the page through the center ray than on the other side in accordance with the deviation direction. By contrast, when there is no deviation from the point of focus, the intensity of the reflected beam will be the same on both sides. The photodetector 9 which detects the distribution of light reflected from the reflecting surface 8 is divided into two sections 9A and 9B at the center ray (optic axis) as shown in plane view in FIG. 6. FIG. 7 indicates the reflected intensities Rp and Rs of p-polarized light and s-polarized light when the refractive index of the detector prism 7 is 1.50. The reflected intensity of unpolarized light is intermediate between the two values, being equal to (Rp+Rs)/2.
If the information-carrying medium 6 is displaced in direction a in FIG. 6, of the light incident on the reflecting surface 8, that part which is located below the center ray in the drawing, extending out to the incident ray a.sub.i1, will all be incident at less than the critical angle, so part of it will become a pencil of transmitted rays bounded by the ray n and the ray a.sub.t1. The intensity of the bundle of reflected rays from the center ray to the outermost reflected ray a.sub.r1 will be reduced by an amount equivalent to the transmitted rays. That part of the light incident on the reflecting surface 8 which is located above the center ray in the drawing, extending out to the incident ray a.sub.i2, will be incident at an angle greater than the critical angle, so none of it will be transmitted; all of it will be reflected into the bundle of reflected rays from the center ray to the outermost reflected ray a.sub.r2. As a result, section 9A of the photodetector 9 will be darkened, while section 9B will be brightened. Section 9B will not brighten if the reflecting surface 8 of the detector prism 7 is set precisely at the critical angle, for the reflection will then simply remain total as can be seen from FIG. 7, but if the reflecting surface 8 is set at slightly less then the critical angle, section 9B will brighten.
As FIG. 7 indicates, the slope of the Rp and Rs curves reaches infinity (in theory) at the critical angle, hence the sensitivity near the point of focus is greatest if the reflecting surface 8 is set at exactly the critical angle. If the reflecting surface 8 is set at less than the critical angle, sensitivity is reduced. If the reflecting surface 8 is set at more than the critical angle, there will be a dead band in which no changes in reflected intensity occur.
If the information-carrying medium 6 is displaced in direction b, the obliquity of the light incident on the reflecting surface 8 will be exactly opposite to that in the discussion above, so the darkening and brightening relationship of sections 9A and 9B of the photodetector 9 will be reversed. Let b.sub.r1, b.sub.r2 and b.sub.t2 denote rays reflected and transmitted by the reflecting surface 8 in this case. When the focus is correct, equal intensities of light will strike sections 9A and 9B of the photodetector 9.
Accordingly, by detecting the difference between the outputs from sections 9A and 9B and determining the amount and polarity of the difference, it is possible to derive a signal representing the amount of deviation of the information-carrying medium 6 from the point of focus, and the direction of the deviation. The focus can then be corrected by moving the objective lens 5 parallel to its optic axis.
The structure of the focus detection apparatus of the prior art as described above gives rise to the following problems. To adjust the apparatus so that when the focus is correct the light will strike the reflecting surface 8 of the detector prism 7 at the critical angle, it is necessary to rotate the prism in the plane of the page in FIG. 6. As FIG. 7 indicates, the reflectivity characteristic changes abruptly in the vicinity of the critical angle, with a discontinuity at the critical angle, so the prism adjustment must be extremely precise. If it exceeds the critical angle even slightly, there will be a dead band in the focusing error signal. If the prism is adjusted to less than the critical angle, however, the abrupt change of the reflectivity characteristic will tend to cause variations in the initial characteristics of the focusing error signals in different manufactured units.
An example of variation that is likely to occur is the sensitivity variation of the focusing error signals in the neighborhood of the correct focus state as is mentioned earlier.
Temperature characteristics and aging changes can cause potentially larger variations in the focusing error signals from the photodetector 9, due to changes in the angle at which the beam enters the detector prism 7, resulting from slight positional displacements of the detector prism 7 or other optical components. As can be seen from the reflectivity characteristic near the critical angle in FIG. 7, if the displacement reduces the angle of incidence, the abrupt change in the reflectivity characteristic will tend to alter the characteristics of the focusing error signal, while if the displacement increases the angle of incidence, the flatness of the reflectivity characteristic above the critical angle will cause a dead band in the focusing error signal.